I. Field of the Invention
This invention relates to a technique for transmitting compressed quantized image-data from a transmitter to a receiver over a limited bandwidth communication channel, which technique employs temporal differential pulse code modulation (DPCM) and data-compression (entropy-reducing) encoding. More particularly, the invention relates to such a technique in which the DPCM incorporates pyramid image processers.
II. Description of the Prior Art
In general, it is well known to employ DPCM and image-data compression encoding to transmit image data, such as that derived from a television video signal, over a limited bandwidth communication channel, such as a telephone line or a somewhat wider limited bandwidth channel which is still significantly narrower than that required to transmit in real time an uncompressed standard bandwidth television video signal (e.g., an NTSC television video signal).
There is a substantial cost saving in transmitting the compresed image data of a television video signal over a limited bandwidth communication channel, with the amount of saving rising as the amount of compression rises and, hence, the required bandwidth of the communication channel narrows. However, the price to be paid for this cost saving is that the image displayed at the receiver tends to be degraded by an amount which rises as a direct monotoric function of the amount of a compression of the transmitted image data. The reason for this is that the compression of image data at the transmitter often results in the loss of image information in an amount that corresponds with the amount of compression. Since this lost image information can never be recovered at the receiver, the displayed image is degraded in accordance with this loss. The result is that, in the past, a DPCM, image-data-compressed encoded transmission system capable of displaying at the receiver an image of sufficiently high quality (i.e., sufficiently high spatial resolution and gray scale) to be practical for teleconferencing, permitted only a relatively small amount of image-data compression. Thus, the savings in communication-channel bandwidth and the cost thereof was relatively small. Consequently, the current cost of teleconferencing remains too high to sustain widespread use.
Spatial-frequency spectrum analyzers and synthesizers employing so-called "pyramids" have been developed for processing in delayed real time images represented by a television video signal. In this regard, reference is made to co-pending patent application Ser. No. 596,817, filed Apr. 4, 1984; co-pending currently-allowed patent application Ser. No. 685,239, filed Dec. 21, 1984; and co-pending patent application Ser. No. 774,984, filed Sept. 11, 1985--all of which co-pending patent applications are assigned to the same assignee as the present invention. Patent application Ser. No. 596,817 discloses a Burt Pyramid spectrum analyzer, a filter-subtract-decimate (FSD) pyramid spectrum analyzer, and a Burt Pyramid synthesizer. Currently-allowed patent application Ser. No. 685,239, which is directed to an interlaced digital video input filter/decimator and/or expander/interpolation filter, discloses in some detail an image processing system employing a Burt Pyramid analyzer and synthesizer, together with a time-skew correction delay means required therewith. Patent application Ser. No. 774,984, discloses an FSD pyramid synthesizer.
All of the pyramid spatial frequency spectrum analyzers disclosed in the aforesaid copending patent applications operate to derive a plurality of separate sub-spectra output video signals, wherein each sub-spectrum output video signal below the highest spatial frequency sub-spectrum output video signal exhibits a pixel density which is below that of its immediately next-higher sub-spectrum output video signal. The plurality of separate sub-spectra output video signals is comprised of one or more bandpass video signals, preferably having a nominal spatial-frequency bandwidth of one octave of the spatial-frequency spectrum in each dimension of the image represented by the analyzed video signal, and a low-pass remnant video signal comprised of the spatial frequencies contained in that portion of the spatial frequency spectrum, in each dimension of the image represented by the analyzed video signal, which is below those contained in the lowest bandpass video signal.
The pyramid spatial frequency spectrum synthesizers disclosed in the aforesaid copending patent applications operate to derive a single synthesized video signal from the separate sub-spectra output video signals, which synthesized video signal corresponds to the original television video signal that has been analyzed.
As is fully explained in the aforesaid co-pending patent application Ser. No. 596,817, such pyramid image processing tends to match the operation of the human visual system in each of four distinct ways. First, the human visual system appears to compute a primitive spatial-frequency decomposition of luminuous images, by partitioning spatial frequency information into a number of contiguous, overlapping spatial-frequency bands, so that spatial frequency information more than a factor of two away from other spatial frequency information is independently processed by the human visual system. Second, the spatial-frequency processing that occurs in the human visual system is localized in space, so that overlapping image subregions that are roughly two cycles wide at each particular spatial frequency are separately processed. Third, the threshold contrast-sensitivity function of the human visual system falls off rapidly as the image spatial frequency increases. Fourth, the ability of the human visual system to detect a change in contrast which is above threshold, is also better at lower spatial frequencies than at higher spatial frequencies.